Thiopodand Linked Squaraine Probes:Synthesis, Properties And Application

Squaraines are a family of extensively studied zwitterionic dyes possessing sharp and intense absorption and fluorescence in the visible to near-infrared (NIR) region. These dyes have often been applied as NIR probes. In the present work, a series of fluorescent probes based on thioether linked squaraine-aniline dyads have been designed and synthesized. All of the new products were fully characterized by FT-IR, ’H NMR,13C NMR, ESI-MS and HRMS, and their optical properties and the sensing behavior toward ions were investigated by absorption and emission spectra.(1) A series of fluorescent probes based on thioether linked squaraine-aniline dyads including unsymmetric squaraine dyes USQ-n (n=1,2,3,4) and symmetric squaraine dyes SQ-n (n=1,2,3,4) have been synthesized. Tioether are extensively used as coordination ligand in the field of chemosensors due to the high affinity with ions. After purification by silica gel column chromatography, these thioether modified squaraines have high purity and stability, and could be used to be designed as fluorescent probes for the detection of ions and small moleculars.(2) Squaraines have a high tendency to aggregate in aqueous solution, and such self-aggregation usually results in a dramatic absorption spectral broadening with fluorescent emission quenching. Researches show that, complexation of the linkage of the probe with ions induces steric hindrance, leading to the disaggregation of the dye aggregates, coupled with a fluorescent emission restoration. Based on this coordination induced disaggregation signaling strategy, USQ-n (n=1,2,3,4) have been developed as sensitive and selective fluorescent probes for the detection of mercury ion. Compared to USQ-1, USQ-2 is highly sensitive to Hg2+, and when tethered to a podand with three sulfur atoms, USQ-3 shows remarkable selectivity and response toward Hg2+. We were pleased to find that the extension of the thioether chain structure improved the recognition ability of our probe toward target ions. USQ-3 was aggregated in pure water solution containing 0.005% TW-80, the addition of Hg2+ to USQ-3 could induce the recovery of fluorescence intensity. Significantly, the aggregated state of squaraine molecules was selectively broken by Hg2+ ions despite a wide range of other interfering metals in water media. Under optimum conditions, the fluorescence recovery is linearly proportional to the concentration of Hg2+ between 0.03 and 1.8 μM and the detection limit is as low as 6.6 nM (1.3 ppb, 3a/k). Further study of the mechanism was formed by absorption and emission spectroscopy, MS,’H NMR and IR titration.(3) Squaraines linked to a flexible thioether chain in a bichromophoric fashion, upon binding with Ca2+, folded, resulting in measurable changes in absorption spectrum and fluorescence quenching. Due to the high affinity between oxalate ions and Ca2+, the addition of oxalate ions could remove Ca2+ and release squaraines, coupled with a fluorescent emission restoration. Based on this strategy, SQ-3 showed good recognition ability for these two ions.